1. Field of the Invention
The present invention relates to a method for measuring tritium and carbon-14 contained in an air sample and an apparatus for performing the same method.
2. Description of the Prior Art
Tritium and carbon-14 contained in air may, in general, exist in various chemical structures and, therefore, in order to measure the amount of each nuclide precisely, it is convenient to integrate these chemical shapes by using a suitable chemical processing.
As a monitor of .sup.3 H and .sup.14 C contained in air, an ionization chamber has been used widely. This is because it is possible to fabricate an ionization chamber having a large volume in comparison with radioactive detectors of other known type. That is, where there is a large amount of sample having low specific radioactivity, i.e., for example, where air is contaminated with a relatively small amount of radioactive substances, it is necessary to collect a large volume of contaminated air in a detector having large volume to measure the radioactivity of the pollutant. For this reason, an ionization chamber whose volume is generally 5-15 liters has been used heretofore as an ordinary environment monitor. However, in the radioactive measurement using ionization chamber, where water and/or other substances, which would degrade the electrical insulation of the chamber, co-exists in air, the measurement itself become very difficult and it is hardly to expect a precise radioactive measurement. To this end, sample is sufficiently dried by using desiccating agent such as silica gel etc. and then introduced into an ionization chamber, or a filter suitable to neutralize ions in air is used. Therefore, there is no problem in a case where the radioactive substances included in the sample has chemical structure which can not be collected by the silica gel etc. used in the preceding stage. However, in a case where the radioactive substances have structures which possibly be collected by the silica gel etc., the measured radioactive value will become unreasonably low. That is, a negative error will be introduced. By considering, particularly, the facts that most of tritium existing in environment takes the form of water and that, where a volatile, labelled compounds are exhausted from an installation in which radioactive substances are handled, the compounds may easily be adsorbed irreversibly by silica gel etc., it can be said that the conventional ionization chamber type monitor was unacceptable. Further, with the ionization chamber type monitor, erroneous radioactive measurement due to smokes etc. is performed resulting in a positive error.
In another method for monitoring tritium in air, which has been recently developed, the water content in air is absorbed by silica gel etc. and then the radioactivity thereof is measured by the liquid scintillation counting method. This method improves the ionization chamber monitor partially. However, the method wherein the water content in air is collected by a desiccating agent such as silica gel etc. and the radioactivity thereof is measured (hereinafter, referred to as condensation method) is based on the general assumption that the object to be measured is in the form of water almost completely. However, since substances other than water, for example, most of organic substances, would be adsorbed at normal temperature, a positive error will be produced if the radioactivity of water is measured by the condensation method, and it can not be considered that all tritium in air is measured. This is because a monitoring for substances which will pass through silica gel without absorption is theoretically impossible.
On the other hand, in a case where the ionization chamber method and the condensation method are merely combined, it is possible for radioactivities of .sup.3 H, .sup.14 C as a whole to obtain a better result than that obtainable by using these two methods individually. However, the information obtainable from the combination of the two methods is essentially a sum of the radioactivities, and as informations about the amount of radioactivity for each of nuclides .sup.3 H and .sup.14 C can not be provided at all. This is because, it is impossible to distinct between .sup.3 H and .sup.14 C in the ionization chamber system. Furthermore, in the case of the combination of the conventional methods, if other radioactive substances than .sup.3 H and .sup.14 C are contained in a sample, the total radioactivity of the other substances will be added to those of .sup.3 H and .sup.14 C, resulting in a positive error for the measurement of .sup.3 H and .sup.14 C.